Semiconductors are used in most electronic appliances. The conventional use of semiconductors has included providing electrical components for printed circuit boards including e.g. transistors and integrated circuits such as memories and processors. The semiconductors are usually manufactured on a silicon substrate. Other semiconducting, conductive and isolating materials, possibly also optical materials are produced on the silicon substrate, and functional semiconductor circuits/components are made by patterning and wiring the layers.
However, new kinds of needs have evolved for semiconductors. Electronic flat displays and other modern user interfaces require large-sized circuits which should be light in weight and robust for everyday handling. The semiconductor circuits which are based on silicon substrates are, however too costly for large-sized applications, and they are not as robust as required.
It has been suggested to produce semiconductor circuitry by using other, low-cost materials as a substrate and providing semiconducting materials as thin film layers. The substrate could be e.g. glass or plastic, or even fiber materials. This kind of technology would allow large-sized semiconductor circuits which have less weight and are not as sensitive to mechanical stresses. It would also be possible to use other semiconducting materials which have special properties. However, with the present technologies it has not been possible to produce material layers of sufficiently high quality, in large sizes and in industrial volumes. Also, the non-homogeneity of semiconducting layers causes non-ideal performance of the semiconductors.
The applicant has investigated possibilities for using laser cold ablation in production of semiconductors. In the recent years, considerable development of the laser technology has provided means to produce very high-efficiency laser systems that are based on semi-conductor fibres, thus supporting advance in so called cold ablation methods. Cold ablation is based on forming high energy laser pulses of short duration, such as within picosecond range, and directing the pulses into the surface of a target material. A plume of plasma is thus ablated from the area where the laser beam hits the target. The applications of cold ablation include e.g. coating and machining.
When employing novel cold-ablation, both qualitative and production rate related problems associated with coating, thin film production as well as cutting/grooving/carving etc. has been approached by focusing on increasing laser power and reducing the spot size of the laser beam on the target. However, most of the power increase was consumed to noise. The qualitative and production rate related problems were still remaining although some laser manufacturers resolved the laser power related problem. Representative samples for both coating/thin film as well as cutting/grooving/carving etc. could be produced only with low with repetition rates, narrow scanning widths and with long working time beyond industrial feasibility as such, highlighted especially for large bodies.
Because the energy content of a pulse, the power of the pulse increases in the decrease of the pulse duration, the problem significance increases with the decreasing laser-pulse duration. The problems occur significant even with the nano-second-pulse lasers, although they are not applied as such in cold ablation methods.
The pulse duration decrease further to femto or even to atto-second scale makes the problem almost irresolvable. For example, in a pico-second laser system with a pulse duration of 10-15 ps the pulse energy should be 5 μJ for a 10-30 μm spot, when the total power of the laser is 100 W and the repetition rate 20 MHz. Such a fibre to tolerate such a pulse is not available at the priority date of the current application according to the knowledge of the writer at the very date.
The prior art laser treatment systems most often include optical scanners which are based on vibrating mirrors. Such an optical scanner is disclosed in e.g. document DE10343080. A vibrating mirror oscillates between two determined angles relative to an axis which is parallel to the mirror. When a laser beam is directed to the mirror, it is reflected with an angle which depends on the position of the mirror at that moment. The vibrating mirror thus reflects or “scans” the laser beam into points of a line at the surface of a target material.
An example of a vibrating scanner or “galvano-scanner” is illustrated in FIG. 1a. It has two vibrating mirrors, one of which scans the beam relative to X-axis and another scans the beam relative to orthogonal y-axis.
The production rate is directly proportional to the repetition rate or repetition frequency. On one hand the known mirror-film scanners (galvano-scanners or back and worth wobbling type of scanners), which do their duty cycle in a way characterized by their back and forth movement, the stopping of the mirror at the both ends of the duty cycle is somewhat problematic as well as the accelerating and decelerating related to the turning point and the related momentary stop, which all limit the utilizability of the mirror as scanner, but especially also to the scanning width. The present coating methods employing galvano-scanners can produce scanning widths at most 10 cm, preferably less. If the production rate were tried to be scaled up, by increasing the repetition rate, the acceleration and deceleration cause either a narrow scanning range, or uneven distribution of the radiation and thus the plasma at the target when radiation hit the target via accelerating and/or decelerating mirror.
Conventionally galvanometric scanners are used to scan a laser beam with a typical maximum speed of about 2-3 m/s, in practice about 1 m/s. If trying to increase the coating/thin film production rate by simply increasing the pulse repetition rate, the present above mentioned known scanners direct the pulses to overlapping spot of the target area already at the low pulse repetition rates in kHz-range, in an uncontrolled way. With repetition rate of 2 MHz even 40-60 successive pulses are overlapping. The overlapping of spots 111 in such a situation are illustrated in FIG. 1b. 
At worst, such an approach results in release of particles from the target material, instead of plasma but at least in particle formation into plasma. Once several successive laser pulses are directed into the same location of target surface, the cumulative effect seems to erode the target material unevenly and can lead to heating of the target material, the advantages of cold ablation being thus lost.
The same problems apply to nanosecond range lasers, the problem being naturally even more severe because of the long lasting pulse with high energy. Here, the target material heating occurs always, the target material temperature being elevated to approximately 5000 K. Thus, even one single nanosecond range pulse erodes the target material drastically, with aforesaid problems.
In the known techniques, the target may not only ware out unevenly but may also fragment easily and degrade the plasma quality. Thus, the surface to be coated with such plasma also suffers the detrimental effects of the plasma. The surface may comprise fragments, plasma may be not evenly distributed to form such a coating etc. which are problematic in accuracy demanding application, but may be not problematic, with paint or pigment for instance, provided that the defects keep below the detection limit of the very application.
The present methods ware out the target in a single use so that same target is not available for a further use from the same surface again. The problem has been tackled by utilising only a virgin surface of the target, by moving target material and/or the beam spot accordingly.
In machining or work-related applications the left-overs or the debris comprising some fragments also can make the cut-line un even and thus inappropriate, as the case could for instance in flow-control drillings. Also the surface could be formed to have a random bumpy appearance caused by the released fragments, which may be not appropriate in manufacturing of semiconductors.
In addition, the mirror-film scanners moving back and forth generate inertial forces that load the structure it self, but also to the bearings to which the mirror is attached and/or which cause the mirror movement. Such inertia little by little may loosen the attachment of the mirror, especially if such mirror were working nearly at the extreme range of the possible operational settings, and may lead to roaming of the settings in long time scale, which may be seen from uneven repeatability of the product quality. Because of the stoppings, as well as the direction and the related velocity changes of the movement, such a mirror-film scanner has a very limited scanning width so to be used for ablation and plasma production. The effective duty cycle is relatively short to the whole cycle, although the operation is anyway quite slow. In the point of view of increasing the productivity of a system utilising mirror-film scanners, the plasma making rate is in prerequisite slow, scanning width narrow, operation unstable for long time period scales, but yield also a very high probability to get involved with unwanted particle emission in to the plasma, and consequently to the products that are involved with the plasma via the machinery and/or coating.
Neither recent high-technological coating methods, nor present coating techniques related to laser ablation either in nanosecond or cold ablation range (pico-, femto-second lasers) can provide any feasible method for industrial scale products comprising larger surfaces. The present CVD- and PVD-coating technologies require high-vacuum conditions making the coating process batch wise, thus non-feasible for industrial scale production of semiconductors. Moreover, the distance between the material to be coated and the coating material to be ablated is long, typically over 50 cm, making the coating chambers large and vacuum pumping periods time- and energy-consuming. Such high-volume vacuumed chambers are also easily contaminated with coating materials in the coating process itself, requiring continuous and time-consuming cleaning processes.
While trying to increase the production rate in present laser-assisted coating methods, various defects such as short circuiting defect factors, pinholes, increased surface roughness, decreased or disappearing optical transparency in optical implementations, particulates on layer surface, particulates in surface structure affecting corrosion pathways, decreased surface uniformity, decreased adhesion, etc. take place.
Plasma related quality problems are demonstrated in FIGS. 2a and 2b, which indicate plasma generation according to known techniques. A laser pulse 214 hits a target surface 211. As the pulse is a long pulse, the depth h and the beam diameter d are of the same magnitude, as the heat of the pulse 214 also heat the surface at the hit spot area, but also beneath the surface 211 in deeper than the depth h. The structure experiences thermal shock and tensions are building, which while breaking, produce fragments illustrated F. As the plasma may be in the example quite poor in quality, there appears to be also molecules and clusters of them indicate by the small dots 215, as in the relation to the reference by the numeral 215 for the nuclei or clusters of similar structures, as formed from the gases 216 demonstrated in the FIG. 2b. The letter “o”s demonstrate particles that can form and grow from the gases and/or via agglomeration. The released fragments may also grow by condensation and/or agglomeration, which is indicated by the curved arrows from the dots to Fs and from the os to the Fs. Curved arrows indicate also phase transitions from plasma 213 to gas 216 and further to particles 215 and increased particles 217 in size. As the ablation plume in FIG. 2b can comprise fragments F as well as particles built of the vapours and gases, because of the bad plasma production, the plasma is not continuous as plasma region, and thus variation of the quality may be met within a single pulse plume. Because of defects in composition and/or structure beneath the deepness h as well as the resulting variations of the deepness (FIG. 2a), the target surface 211 in FIG. 2b is not any more available for a further ablations, and the target is wasted, although there were some material available.